A brazing method has been widely used as a method for joining parts having a large number of small joints (e.g., aluminum heat exchanger and machine parts). When brazing aluminum (including an aluminum alloy), it is indispensable to break the oxide film that covers the surface of the material so that the molten filler metal comes in contact with the matrix or another molten filler metal. The oxide film may be broken by utilizing a method that utilizes flux or a vacuum heating method. These methods have been put to practical use.
The brazing method has been applied to various fields. The brazing method has been most typically applied to automotive heat exchangers. Most automotive heat exchangers (e.g., radiator, heater, condenser, and evaporator) are made of aluminum, and produced by applying the brazing method. A method that applies a non-corrosive flux to the material, followed by heating in nitrogen gas is most widely used at present.
In recent years, a heat exchanger provided with electronic parts (e.g., inverter cooler) has been used along with a change in driveline (e.g., electric car and hybrid car), and a flux residue has posed problems. Therefore, some of the inverter coolers are produced using a vacuum brazing method that does not utilize flux. However, since the vacuum brazing method utilizes a heating furnace that increases the equipment and maintenance costs, and has problems as to productivity and brazing stability, a brazing method that is implemented in a nitrogen gas furnace without using flux has been increasingly desired.
For example, a method that utilizes a brazing sheet that is clad with a filler metal to which a small amount of Be is added, and performs an etching treatment in an acid or an alkali, followed by heating for brazing was put to practical use. However, the application range of this method was limited, and this method is rarely used at present. The application range of this method was limited for the following reasons.
(1) Since brazability is poor as compared with the method that applies a flux, a brazing failure easily occurs.
(2) A toxic element (i.e., Be) is included in the filler metal, even in a small amount.
The problem (1) is a fundamental problem. For example, even if easy brazing (e.g., brazing a fin and a tube) can be performed without a problem, it may be difficult to reliably braze an area where it is necessary to prevent leakage (e.g., an area in which a tube is inserted into a header, or a joint at the outer circumference of a hollow heat exchanger formed using a pressed sheet). Therefore, the application range is limited to a heat exchanger that can be easily brazed (e.g., a stacked-type heat exchanger that is mainly brazed in a plane, or a heat sink that is produced by brazing a fin to a base such as an extruded shape). The problem (2) (i.e., the toxicity of Be) is serious in the fields of food, medical equipment, and automotive heat exchangers, and the above method may generally be rejected for this reason.
In order to solve the above problems, the inventors of the invention proposed a brazing sheet in which a filler metal includes 0.004 to 0.1% of Li (see Japanese Patent Application No. 2012-105797). However, it was found through further studies that the surface oxide film must be removed from the brazing sheet by an etching treatment before heating for brazing in order to uniformly braze a heat exchanger having a complex joint structure.
Specifically, when producing a heat exchanger in which different joints are present at a short distance using a brazing method that utilizes a flux, the oxide film is broken and removed due to the flux, and a fillet is uniformly formed at each joint (i.e., the filler metal does not unevenly flow between the joints). However, the molten filler metal is drawn toward each joint when using a brazing method that does not utilize a flux. As a result, the molten filler metal is preferentially drawn in the direction in which a fillet is easily formed (i.e., the filler metal unevenly flows), and a brazing failure easily occurs at some joints.
Therefore, in order to implement a brazability equal to that achieved by brazing using flux when implementing brazing without using flux, it is necessary to remove the oxide film via an etching treatment before heating for brazing. However, since the etching treatment has a problem in connection with waste disposal, and waste management/disposal increases the cost, it is generally desired to avoid the etching treatment on the brazing site. This problem may be solved by shipping a material that has been subjected to the etching treatment on the material production site. However, the etching effects may be lost during storage, and a decrease in brazability may occur. (Japanese Patent No. 994051, JP-A-11-285817)